Eddy current testing is one method of nondestructive testing that uses electromagnetic wave phenomena. Like all such methods, the testing is indirect: the material properties to be measured must be correlated with appropriate electromagnetic properties. This correlation succeeds, provided that the test conditions are well controlled and that the instrumentation is well calibrated.
Eddy current testing involves the observation of the interaction between electromagnetic fields and metals. The basic requirements are a coil or coils carrying an alternating current, to which an electrically conductive specimen is subjected, and a means of measuring the response current or voltage. The alternating current through the coil, of a chosen frequency, produces eddy currents in the specimen, which in turn produce fields in the coil.
One means of applying the current to the specimen is holding the test coil in a probe that is moved over the surface of the specimen. A part of this test process is maintaining the correct angle between the specimen and the coil. Ideally, this relationship is a constant, and analysis is easiest if the coil is kept perpendicular to the surface of the specimen. For flat surfaces, this goal is easily met, but for surfaces having irregular shapes, the probe must be adjusted according to surface variations. For example, many applications involve testing parts of equipment that may have irregular surface features, such as curves, bolt holes, flanges and the like.
In application, the basic method of testing involves moving the probe over the specimen surface. Sophisticated automated systems have been developed for providing the exciting current, moving the probe, and analyzing the response signal.
To maintain a constant angle between the coil and the test surface, existing test systems use a set of probes designed to access all possible surfaces. For example, one probe might be designed for flat surfaces, whereas another probe might be designed for reaching into a bolt hole or around a corner of a flange. As the test arm is moved over the specimen, appropriate probes are installed. Mechanical manipulators can be used to vary the angle to some extent, but the range of manipulation is limited, and inadequate for some test geometries.
A problem with using multiple probes is that each probe must be recalibrated once installed. This is due to the fact that each eddy current coil is somewhat different in electromagnetic properties. This recalibration results in substantial overhead in terms of time during the test process. In some irregular test surface geometries, the total inspection time may be dominated by the calibration time, rather than the actual test time. Thus, a need exists for a method of eddy current testing that eliminates the need for multiple probes.